accidental triggering of the residual-current-operated circuit breaker. This has
to be taken into account
●
for switch-on processes, due to high asymmetric inrush currents and
●
during operation of the installation, due to leakage currents produced in
normal operation.
Cause of leakage currents
For the purpose of stepless speed variation with a high degree of positioning
accuracy and dynamic response, certain modulation procedures are neces‐
sary for drive systems. For physical reasons, these modulation procedures
give rise to inevitable leakage current produced during normal operation. Es‐
pecially with unbalanced loads of the mains phases or a large number of
drives it can easily reach some amperes (rms value).
The leakage current is not sinusoidal but pulse-shaped. For this reason,
measuring instruments normally sized for alternating currents in the range of
50 Hz are not suited. Use measuring instruments with rms value measuring
ranges up to at least 150 kHz.
The degree of leakage current depends on the following features of the in‐
stallation:
●
Type of inrush current limitation
●
Number, type and size of drives used
●
Length and cross section of connected motor power cables
●
Grounding conditions of the mains at the site of installation
●
Unbalance of the three-phase system
●
Types of filters and chokes connected in the incoming circuit
●
EMC measures that are taken
If measures are taken to improve the electromagnetic compatibility (EMC) of
the installation (mains filters, shielded lines), the leakage current in the
ground wire is inevitably increased, especially when switching on or in the
case of mains unbalance. Given these operating conditions, residual-current-
operated circuit breakers can trigger without an error having occurred.
The EMC measures are mainly based on capacitive short-circuiting of the in‐
terference currents within the drive system. Inductive filter measures can re‐
duce the leakage currents, but affect the dynamic response of the drive and
bring about
●
higher construction volume
●
higher weight
●
expensive core material
Possibilities of use
Motor cable lengths
Keep the motor cables as short as possible. Only short motor cables do allow
low leakage currents and thereby enable residual-current-operated circuit
breakers to work.
Types of residual-current-operated
circuit breakers
There are two types of residual-current-operated circuit breakers:
1.
Residual-current-operated circuit breakers sensitive to power pulse cur‐
rent (type A acc. to IEC 60755)
These are normally used. However, it is only pulsating direct fault cur‐
rents of a maximum of 5 mA and sinusoidal alternating fault currents
that they switch off safely. This is why they are not allowed for devices
that can generate smoothed direct fault currents. In the case of smooth‐
DOK-INDRV*-HXX05******-PR02-EN-P
Bosch Rexroth AG
103/407
Rexroth IndraDrive ML Drive Systems with HMU05
Combining the individual components
